The increase demand for higher areal recording densities imposes increasingly greater demands on longitudinal magnetic recording media in terms of remanent coercivity (Hr), magnetic remanence (Mr), coercivity squareness (S*), signal-to-medium noise ratio (SMNR), and narrow track recording performance. It is extremely difficult to produce magnetic recording media that satisfies all or most of these requirements.
Longitudinal recording media is structured as a layered material of films deposited on a substrate. The recording media typically has one or more underlayers, such as a chromium (Cr) or a Cr alloy film, one or more magnetic layers, such as a cobalt (Co) alloy, and a protective overcoat. The Co alloy magnetic layer typically contains polycrystallines grown on a polycrystal Cr or Cr alloy underlayer. The underlayer, magnetic layer, and protective overcoat are typically deposited by physical vapor deposition techniques, such as sputtering.
A magnetic material is composed of a number of submicroscopic regions called domains. Each domain contains parallel atomic magnetic moments but the directions of magnetization of different domains are not necessarily parallel. In the absence of an applied magnetic field, adjacent domains may be oriented randomly in any number of several directions, often called the directions of easy magnetization, which depend on the geometry of the crystal. When a magnetic field is applied, many of the domains may rotate and align parallel to the applied field. Also, the domains most nearly parallel to the direction of the applied field may grow in size at the expense of the others. This is called boundary displacement of the domains or domain growth. When the material reaches the point of saturation magnetization, no further domain growth occurs, even if the magnitude of the external magnetic field is increased.
Magnetic properties, such as remanent coercivity (Hr), remanent magnetization (Mr) and coercive squareness (S*), which are important to the recording performance of the recording media, depend in part on the microstructure of the Co alloy magnetic film for a given Co alloy composition. For longitudinal magnetic recording media, the desired crystalline structure of the Co and Co alloys is hexagonal close packed (HCP) with uniaxial crystalline anisotropy and a magnetization easy direction along the c-axis in the plane of the film. The better the in-plane c-axis crystallographic texture is, the higher the remanent coercivity of the Co-alloy magnetic film is.
The grain size of the magnetic film also affects the magnetic performance of recording media. Remanent coercivity increases with an increase in grain size in certain range of grain size, however, the larger the grain size is, the higher the medium noise level of the recording media is, that is, the lower the SMNR is. Thus, there exists a need to achieve high remanent coercivities without the increase in medium noise associated with relatively large grain size. To achieve a low noise recording medium, the Co alloy magnetic layer should have fairly uniform and small grain size with grain boundaries that can magnetically isolate neighboring grains. This kind of microstructure and crystallographic texture is normally achieved by manipulating the deposition process, by grooving the non-magnetic substrate surface, which is referred as mechanical texturing, or most often by the proper use of one or more underlayers with a preferred crystallographic orientation.
The linear recording density can be increased by increasing the remanent coercivity and/or by decreasing the medium noise of the recording medium. This can be accomplished by producing a magnetic layer with fine, magnetically non-coupled grains. Medium noise in thin films is a dominant factor restricting increased recording densities, and is attributed in part to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
Recording performance is also determined by media properties known as PW50 and overwrite (OW). PW50 is the half width of the output signal, that is, the width of that portion of a pulse from the time its rising edge reaches one half of its amplitude to the time its falling edge falls to one half of its amplitude. A wide PW50 indicates that adjacent bits are crowded together resulting in interference, which limits the linear packing density of bits in a given track. Means of reducing PW50 include reducing Mrt (magnetic film thickness, t, times magnetic remanence, Mr), raising Hr, and increasing S*.
OW is a measure of what remains of a first signal after a second signal (for example of a different frequency) has been written over it on the media. Recording media with relatively poor OW characteristics maintains a good portion of the first signal after erasure. OW is improved by raising S* and by decreasing Hr and Mrt.
Li-Lien Lee et al. disclosed NiAl underlayers, which have small grain size, and promote (10.0) crystallographic orientation of the magnetic media, (“NiAl underlayers for CoCrTa magnetic thin films”, IEEE Transaction on Magnetics, Vol. 30, No. 6, pp. 3951-3953, November 1994, and U.S. Pat. No. 5,693,426). Seagate's co-pending patent application, SEA 2758, filed on Sep. 10, 1999, and entitled “Magnetic Recording Medium with a NiAlRu seedlayer,” discloses NiAlRu seedlayers, which also promote (10.0) crystallographic orientation of the magnetic media. “Seedlayer Induced (002) Crystallographic Texture in NiAl Underlayers,” L.-L. Lee, D. E. Laughlin and D. N. Lambeth, J. Appl. Phys., 79 (8), pp. 4902-4904 (1996), discloses a MgO seedlayer, which induces Cr(200) preferred orientation. “FeAl Underlayers for CoCrPt Thin Film Media,” L.-L. Lee, D. E. Laughlin and D. N. Lambeth, J. Appl. Phys., 81 (8), pp. 43664368 (1997), first reported an FeAl underlayer having a B2 structure.
U.S. Pat. No. 6,174,582 discloses a seedlayer containing a refractory metal that promotes a (200) orientation in the Cr underlayer and a (11 {overscore (2)} 0) orientation in the magnetic layer. The refractory metal can be selected from tantalum, niobium, vanadium, tungsten, molybdenum, or chromium. U.S. Pat. No. 6,156,404 discloses an underlayer with a B2 crystal structure. This underlayer encourages a subsequent chromium layer to grow in a manner other than with a (200) orientation. In one case, the underlayer induces the chromium underlayer to grow with a preferred (110) orientation. The materials that can be used as an underlayer include a ruthenium-aluminum alloy. The materials that can be used for the chromium underlayer include chromium or a chromium alloy such as an alloy containing tantalum, vanadium, or molybdenum.
In order to store as much digital information as possible on a recording medium there is a continuing need for improved areal density magnetic recording media exhibiting high remanent coercivity and high SMNR. It is also desirable to produce recording media that has a minimum PW50. The need for lighter, smaller and better performing computers with greater storage density demands higher density recording media. The present invention satisfies these demands with a longitudinal magnetic recording media having high remanent coercivity and low medium noise.
Media with Co(10.0) preferred orientations have narrower in-plane C-axis dispersion than that of the media with Co(10.0) preferred orientations. Oriented magnetic media having Cr-containing underlayers with cubic (200) crystallographic orientations and Co(11.0) crystallographic orientations have better recording performances than isotropic media having Co(10.0) crystallographic orientations.
Oriented magnetic media with Mrt orientation ratio (OR-Mrt) of about 1.5 have about 2.5 dB higher media signal-to-noise ratio (SMNR) tested at 500 kfci (thousand flux reversals per inch) than isotropic media, which have OR-Mrt of 1. Mrt orientation ratio stands for the ratio of Mrt along the circumferential direction over Mrt along the radial direction. Mrt stands for product of remanent magnetization and magnetic film thickness. The Mrt orientation ratio of more than 1.05 is caused by the combination of circumferential mechanical texturing/grooving of the substrate and the Co(11.0) crystallographic orientation. Oriented media are the media with OR-Mrt more than 1, e.g. more than 1.05. Deeper grooves usually induce higher orientation ratio, but also increase film roughness and higher flight height of the magnetic heads will not be avoided. High flight height is not desirable. There is, however, a need to find other underlayer materials, which promote Co(11.0) crystallographic orientations and good recording performances of magnetic media. There is also a need to find a method, which can be used to make an oriented medium with good recording performances at low OR-Mrt, less than 1.2.